In recent years, fluorine-containing compounds have been noticed in various fields of applications such as medicines and electronic materials because of peculiar properties thereof, and have been used in numerous application fields by utilizing their functions attributable to fluorine atoms. Therefore, various methods for effectively introducing fluorine atoms into compounds have been studied. Conventionally known fluorination techniques include, for example, a direct fluorination method using a fluorine gas, a so-called halogen exchange method in which a compound containing a halogen atom such as chlorine and bromine is reacted with HF or KF to replace the halogen atom with fluorine, a method using a combination of hydrogen fluoride with a base such as pyridine and triethylamine, a method using a hypervalent iodine, e.g., IF5, an electrolytic fluorination method, etc. In the present invention, the “halogen” means halogens other than fluorine unless otherwise specified.
Among these methods, the halogen-fluorine exchange method has been extensively used because fluorine atoms can be introduced relatively easily and aimed fluorine-containing compounds can be produced easily, although the use of halogen compounds as the starting materials is somewhat undesirable. The details about the techniques for producing fluorine compounds by the halogen-fluorine exchange method are described, for example, in Adv. Fluorine Chem., 1963, (3), p. 181. This document describes the halogen-fluorine exchange reaction between halogen-containing aliphatic compounds, aromatic compounds, branched aromatic compounds, heterocyclic compounds, carboxylic acids, sulfonic acids, silicon compounds or phosphorus compounds with fluorine compounds such as HF, KF, SbF3 and SbF5.
The halogen-fluorine exchange reaction requires a halogenated precursor corresponding to the aimed compound. There are also many well known techniques for introducing halogens to produce such a halogenated precursor, for example, a method using chlorine gas, iron chloride, phosphorus trichloride, phosphorus pentachloride, thionyl chloride, sulfuryl chloride, aluminum chloride, chloroform, carbon tetrachloride, titanium chloride, phosgene, N-chlorosuccinimide, etc.; and a method using zeolites such as zeolite X, Y, L and β and mordenite together with chlorine gas, thionyl chloride, sulfuryl chloride or the like for improving selectivity of the halogenation.
In the photochlorination using chlorine gas under irradiation of light, even low-reactive molecules such as methane are effectively converted into methyl chloride by a free-radical chain reaction. This method is useful for chlorinating side chains of aliphatic or aromatic compounds. For example, Kirk-Othmer, “Encyclopedia of Chemical Technology”, 4th edition, describes the production of benzyl chloride from toluene as an example of the photochlorination of side chain of aromatic compounds.
Not Limited merely to aliphatic or aromatic hydrocarbons, the photohalogenation is widely applied to various compounds such as heterocyclic compounds, carboxylic acids, sulfonic acids, silicon compounds and phosphorus-containing compounds.
For example, JP 2001-294551 A discloses a method for producing 3,5-bis(trichloromethyl)benzoyl chlorides by the photochlorination of 3,5-dimethylbenzoyl chlorides.
This method has been also disclosed in French Patent No. 820696 (1937). The French Patent teaches a method for chlorinating branched methyl group of methyl-containing benzenecarboxylic acids by chlorine gas, and describes in Example 10 an actual procedure for producing 3,5-bis(trichloromethyl)benzoyl chloride from 3,5-dimethylbenzoyl chloride by introducing chlorine atom under irradiation of light. Further, U.S. Pat. No. 2,181,554 (1939) discloses a method for producing 3,5-bis(trifluoromethyl)benzoyl fluoride from 3,5-bis(trichloromethyl)benzoyl chloride by the halogen exchange. Concerning the photohalogenation, there are so many other documents to make it difficult to comprehensively review them.
The above known photohalogenation methods and halogen-fluorine exchange methods are extensively used for introducing fluorine atoms to various compounds. Since these methods use a halogenated compound as a precursor for introducing fluorine atoms, the final products inevitably contain halogens as impurities. The impurities such as residual halogens, residual metals and residual alkali or alkaline earth metals generally affect adversely in the application fields of medicine and electronic materials, and should be reduced to a ppb level or less in some cases. The residual halogens, for example, in medical applications pyrogen or other trace impurities tend to significantly affect human health and in electronic material applications such as photoresists the residual chlorine causes corrosion of electronic devices. The residual metals, for example, residual Pd migrates to cause defective insulation or defective operation. Sb remaining in chemically-amplified positive photoresists causes a positive-negative reversal during the irradiation of X-ray or electron beams. U, Th, etc., remaining in sealing materials cause soft error. Thus, the residual impurity elements have adverse influences in many cases. Therefore, it has been strongly required to minimize the content of impurities such as residual halogens and residual metals.